The generation of NO2 has become important for the passive regeneration of diesel particulate filters, as well as the enhancement of low temperature SCR (“Selection Catalytic Reduction”) activity. The low temperature activity of conventional diesel oxidation catalysts (DOC) has been derived from the incorporation of platinum group metals (PGM), typically Pt or Pd. With improvements in fuel quality, specifically, the increased availability of ultra low sulfur diesel, the formation of sulfate over highly active DOCs has become less of an issue. As a result, improvements in low temperature activity for HC and CO oxidation could be obtained by increasing PGM loading. Because of cost advantages associated with Pd relative to Pt, utilization of higher Pd levels in DOC formulations has become common. It has also been recognized that Pd is effective in the thermal stabilization of Pt, enhancing the performance of a mixture following high temperature aging. Although Pd can be used effectively for the oxidation of HC and CO, it is not nearly as effective as Pt for the oxidation of NO to NO2. In mixtures of Pt and Pd, the efficiency of NO oxidation is found to decrease with decreasing Pt/Pd ratio.
With the adoption of stricter emission regulations forcing a significant reduction in NOx emissions, advanced diesel combustion strategies have been developed to minimize NOx levels from the engine. Unfortunately, many of these combustion strategies also result in higher engine-out levels of CO and HC, as well as lower exhaust temperatures. This combination has driven the need for lower DOC light-off temperatures to manage CO and HC emissions. This in turn has further increased the use of DOCs with high PGM loadings, with an associated increase in the cost of the DOC.
At the same time, stricter emission regulations are forcing the incorporation of particulate filters to control PM emissions. In many applications, DOCs are being utilized to oxidize NO to NO2. The generated NO2 then serves as an effective low temperature oxidant for soot. As with the oxidation of CO and HC, generation of a higher fraction of NO2 in the exhaust stream is benefited by higher PGM loadings, with again an associated increase in the cost of the DOC.
Because advanced combustion strategies often result in lower engine-out NOx levels while maintaining or even increasing the levels of engine-out particulate, the availability of NO2 to “passively” combust soot at a rate sufficient to prevent accumulation of unacceptable levels of soot within a filter (i.e. unacceptable engine back pressure and associated fuel economy penalty) requires the utilization of other measures to combust the accumulated soot. This type of “desooting” process often referred to as active regeneration can be accomplished by heating the soot accumulated within the filter to the point where oxygen is able to efficiently combust the soot. In many applications, DOCs are being utilized to generate the heat necessary to initiate combustion of the trapped particulate. This in turn has increased the thermal durability requirement of DOCs. Again, this has often resulted in the requirement for an increased PGM loading in order to obtain sufficient low temperature performance in the aged state.
Importantly, there is also a fuel economy penalty associated with the heat generation associated with active filter regeneration. As a result, even though passive filter regeneration may be insufficient by itself to prevent soot accumulation from reaching a point where active filter regeneration is required, the combustion of particulate with NO2 can reduce the rate of soot accumulation. This reduction in soot accumulation rate reduces the frequency at which active regeneration is required, and as a result, lowers the fuel economy penalty associated with filter operation. Again, this drives the use of DOCs with high PGM loadings, and specifically a higher Pt fraction to increase NO2, with an associated increase in the cost of the DOC.
As a result of the tradeoffs in Pt and Pd cost and performance, numerous optimization studies have been conducted in an effort to minimize PGM cost contribution to DOC while maintaining or improving system performance. The application of two catalyst combinations, the first containing high PGM loading and the second containing low PGM loading is known. It is also known that zones or bands of high and low PGM loading can be applied to a single catalyst substrate, providing activity similar to that of two catalyst combinations. Both types of configurations are depicted in FIG. 11. It is also known in the art, that different PGM ratios can be applied in these two catalyst combinations or zoned/banded catalyst designs. While these designs have provided improved activity for HC and CO performance, to date, these designs have had limited success in simultaneously enhancing NO2 generation while minimizing PGM cost.
Devices are known for the purification of diesel exhaust gases, which devices comprise, in the flow direction of the exhaust gas, an oxidation catalyst, a diesel particulate filter with catalytically active coating, and, downstream of a device for introducing a reducing agent from an external reducing agent source, an SCR (“selective catalytic reduction”) catalyst.
The untreated exhaust gas of diesel engines contains, in addition to carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15% by volume. The untreated exhaust gas also contains particulate emissions which are composed predominantly of soot residues and possible organic agglomerates which arise from incomplete fuel combustion in the cylinder.
Adhering to future legal exhaust gas limits for diesel vehicles in Europe, North America and Japan necessitates the simultaneous removal of particulates and nitrogen oxides from the exhaust gas. The harmful gases carbon monoxide and hydrocarbons from the relatively lean exhaust gas can easily be made harmless by oxidation at a suitable oxidation catalyst. Diesel particulate filters with and without an additional catalytically active coating are suitable units for the removal of the particulate emissions. On account of the high oxygen content, the reduction of the nitrogen oxides to form nitrogen (“denitrogenization” of the exhaust gas) is more difficult. A known method is selective catalytic reduction (SCR) of the nitrogen oxides at a suitable catalyst.
This method is presently the preferred option for the denitrogenization of diesel engine exhaust gases. The reduction of the nitrogen oxides contained in the exhaust gas takes place in the SCR method with the aid of a reducing agent which is introduced into the exhaust stream in a dosed fashion from an external source. As reducing agent, use is preferably made of ammonia or of a compound which releases ammonia, such as for example urea or ammonium carbamate. The ammonia, which is possibly generated in situ from the precursor compound, reacts at the SCR catalyst with the nitrogen oxides from the exhaust gas in a comproportionation reaction to form nitrogen and water.
Another suitable method for the denitrogenization of diesel engine exhaust gases utilizes a catalyst which is capable of storing NOx during oxygen rich operating conditions and releasing and reducing the stored NOx during short periods of fuel rich operation. Such devices are known as NOx adsorbers or lean NOx traps (LNTs).
At present, in order to satisfy the upcoming legal standards, a combination of the different exhaust gas purification units is inevitable. A device for the purification of diesel engine exhaust gases must comprise at least one oxidationally active catalytic converter and, for denitrogenization, an SCR catalyst with an upstream device for introducing reducing agent (preferably ammonia or urea solution) and an external reducing agent source (for example an auxiliary tank with urea solution or an ammonia store), or an LNT. If it is not possible by optimizing the combustion within the engine to keep the particulate emissions sufficiently low that they can be removed by means of the oxidation catalyst by direct oxidation with oxygen, the use of a particulate filter is additionally necessary.
Corresponding exhaust gas purification systems have already been described; some are presently at the practical testing stage, others are already commercially practiced.
For example, EP-B-1 054 722 describes a system for the treatment of NO and particulate-containing exhaust gases in which system an oxidation catalyst is connected upstream of a particulate filter. Arranged at the outflow side of the particulate filter are a reducing agent source and a dosing device for the reducing agent, and an SCR catalyst. In the method described in EP-B-1 054 722, the NO2 proportion in the exhaust gas and therefore the NO2/NO ratio is increased by means of the at least partial oxidation of NO at the oxidation catalyst, with the NO2/NO ratio preferably being set to a predetermined level which is an optimum for the SCR catalyst.
The NO2/NO ratio which is an optimum for the SCR catalyst is 1 for all presently known SCR catalysts. If the NOx contained in the exhaust gas is composed only of NO and NO2, then the optimum NO2/NOx, ratio is between 0.3 and 0.7, preferably between 0.4 and 0.6 and is particularly preferably 0.5. Whether said ratio is attained upstream of the SCR catalyst in a system according to EP-B-1 054 722 is dependent on the exhaust gas temperature and therefore on the operating state of the engine, on the activity of the oxidation catalyst and on the design and soot loading of the diesel particulate filter which is connected downstream of the oxidation catalyst.
The untreated exhaust gas of conventional diesel engines contains only a very low proportion of NO2 in the NOx. The main proportion of the nitrogen oxides is nitrogen monoxide NO. As said untreated gas passes over the oxidation catalyst, NO is at least partially oxidized to form NO2. The rate of NO2 formation is dependent on the activity of the oxidation catalyst and on the exhaust gas temperature. If a significant quantity of soot is deposited on the diesel particulate filter which is arranged at the outflow side, then the NO2 proportion present in the NO downstream of the oxidation catalyst is, with sufficient exhaust gas temperature, further reduced. Since NO is predominantly formed from the NO2 during the oxidation of soot with NO2 essentially no denitrogenization of the exhaust gas takes place. As a result, denitrogenization must take place by means of the downstream SCR catalyst, for which purpose the NO2/NOx ratio must be set to an optimum value over the entirety of oxidation catalyst and diesel particulate filter. EP-B-1 054 722, however, does not provide any technical teaching as to how the setting of the NO2/NOx ratio in the exhaust gas upstream of the SCR catalyst can be realized over the entirety of the oxidation catalyst and filter.
A further problem which is not discussed in EP-B-1 054 722 but which occurs in practice is that the “passive” particulate filter regeneration which takes place in the system, that is to say the burning of soot, which takes place in situ, by oxidation with NO2 generated by means of the oxidation catalyst, is generally not sufficient on its own to prevent the particulate filter from becoming clogged with soot, with a resulting rise in exhaust gas back pressure to unacceptable values. Applied auxiliary measures are necessary, which may be carried out by means of for example, additional “active” diesel particulate filter regenerations when the pressure drop across the particulate filter exceeds a critical threshold value.
The auxiliary measures include the additional injection of fuel into the exhaust stream upstream of the oxidation catalyst or into the cylinders of the combustion chamber during the exhaust piston stroke. The unburned fuel which passes into the exhaust gas from time to time by means of said device is burned across the oxidation catalyst with the release of heat; the oxidation catalyst is used as a “heating catalyst” in order to heat the downstream diesel particulate filter to temperatures which lie considerably above the soot ignition temperature in the oxygen-containing atmosphere, that is to say in the range from 500 to 650° C. As a result of the temperature rise which is obtained in this way, the soot particles are “burned off” with the oxygen contained in the exhaust gas.
In order that the oxidation catalyst can operate as a “heating catalyst” in the “active” diesel particulate filter regeneration, the oxidation catalyst must meet some demands with regard to conversion behaviour and ageing stability. The oxidation catalyst must be able to convert high quantities of unburned hydrocarbons by oxidation in a short time without the oxidation reaction thereby being “flooded” and thus ceasing. This is also sometimes referred to as quenching of the catalyst. Here, the conversion of the unburned hydrocarbons must be as complete as possible, since the breakthrough of unburned hydrocarbons through the oxidation catalyst can lead to the contamination of the SCR catalyst which is arranged further downstream. A breakthrough of unburned hydrocarbons at the end of the exhaust system may also have the result that the legal limits are not adhered to. The more fuel can be burned completely across the oxidation catalyst, the more flexible can be the strategy for active regeneration. Furthermore, it is an important requirement that the oxidation catalyst “ignites” even at low exhaust gas temperatures (180 to 250° C.).
An oxidation catalyst which is also ideally suitable as a heating catalyst must therefore provide very high HC conversion rates even at extremely low exhaust gas temperatures, wherein the HC conversion should increase as abruptly as possible to maximum values once the “ignition temperature” (light-off temperature) is reached. Furthermore, the catalyst must be sufficiently stable with regard to ageing that its activity is not impaired to too great an extent as a result of the exothermic energy generated during the combustion of the hydrocarbons. The performance demands are referred to below in summary as “heat-up performance”.
The present invention is intended to provide an exhaust gas purification zoned catalyst system, in which the oxidation catalyst exhibits the best possible “heat-up performance” in the case of an “active” particulate filter regeneration.